Forever clean? Metal–organic 'micromushrooms' repel all

Date:

April 10, 2014

Source:

The Agency for Science, Technology and Research (A*STAR)

Summary:

A clever chemical transformation yields surface-bound microstructures that efficiently drive away oil- and water-based contaminants. Natural surfaces that repel water, such as lotus leaves or butterfly wings, often have a rough, microscale texture that traps air beneath the liquid droplet. By mimicking these biological structures, researchers have developed 'superhydrophobic' coatings that are highly resistant to wetting. One trick unknown to nature, however, is the ability to repel hydrocarbon-based oils that have much lower surface tension than water and tend to spread out rather than bead up.

Natural surfaces that repel water, such as lotus leaves or butterfly wings, often have a rough, microscale texture that traps air beneath the liquid droplet. By mimicking these biological structures, researchers have developed 'superhydrophobic' coatings that are highly resistant to wetting. One trick unknown to nature, however, is the ability to repel hydrocarbon-based oils that have much lower surface tension than water and tend to spread out rather than bead up.

Jia Min Chin and co-workers from the A*STAR Institute of Materials Research and Engineering and A*STAR Institute of Bioengineering and Nanotechnology in Singapore have now discovered a simple procedure to synthesize 'omniphobic' interfaces that repel both oil and water using intricate, mushroom-shaped, metal-organic crystal frameworks.

Recent efforts toward omniphobic surfaces have focused on producing reentrant microscale textures, which have curved shapes that inherently retain air pockets. These structures prevent oil from wetting the surface and stabilize the beaded droplet state. Currently, complicated and labor-intensive lithographic fabrication techniques are needed to generate such textures.

Chin and co-workers investigated a 'bottom-up' strategy to synthesize omniphobic films using metal-organic frameworks (MOFs) -- compounds that connect metal ions into multidimensional structures using hydrocarbon-based linkages. Previous studies have shown that an aluminum-containing MOF, known as NH2-MIL-53(Al), can controllably form micro- and nanoscale rods and needles. The team suspected that suitable synthetic conditions could yield spontaneous needle growth upward from a substrate, forming a micro-rough surface with numerous trapped air pockets.

To achieve this, the researchers mixed their MOF precursor with an aluminum oxide membrane and applied 'hydrothermal' high temperature-high pressure aqueous reaction conditions. This resulted in perpendicularly aligned needles on both sides of the membrane. Next, the team faced the challenge of transforming the needles into curved textures suitable for repelling oil. After many attempts, they spotted an important clue -- the modified membranes 'floated' on top of aqueous surfaces due to their superhydrophobic nature.

Chin and her team exploited this floating effect by suspending the microneedle-covered membrane in an aqueous solution of the MOF precursor. Additional MOF growth occurred only on the wetted tips of the needles, expanding the crystalline stems into mushroom-like caps (see image). By controlling the reaction time to generate a targeted cap size, the researchers' omniphobic surface successfully repelled long-chain hydrocarbon oils.

Chin notes that this benchtop, chemical process produces results previously limited to facilities with expensive, high-tech equipment. "Our aim was to develop simple techniques for fabricating interesting structures which are accessible to scientists around the world," she says.

Aug. 1, 2015  After debuting the world's first solar air battery last fall, researchers have now reached a new milestone. They report that their patent-pending design -- which combines a ... read more

July 29, 2015  Using a hybrid silica sol-gel material and self-assembled monolayers of a common fatty acid, researchers have developed a new capacitor dielectric material that provides an ... read more

July 31, 2015  As the demand grows for ever smaller, smarter electronics, so does the demand for understanding materials’ behavior at ever smaller scales. Physicists are building a unique ... read more

July 31, 2015  Nanoscale worlds sometimes resemble macroscale roller-coaster style hills, placed at the tip of a series of hexagons. Surprisingly, these nanohills stem from the self-organization of particles -- the ... read more

July 31, 2015  Precise targeting biological molecules, such as cancer cells, for treatment is a challenge, due to their sheer size. Now, scientists have proposed an advanced solution that can potentially be applied ... read more

July 30, 2015  The behavior of fruit flies, which are commonly used in laboratory experiments, is altered by electric fields, new research shows. The research indicates that the wings of the insects are disturbed ... read more

May 29, 2013  Paper is known for its ability to absorb liquids. But by modifying the underlying network of cellulose fibers, etching off surface “fluff” and applying a thin chemical coating, researchers have ... read more

Oct. 15, 2012  The colors of a butterfly's wings are unusually bright and beautiful and are the result of an unusual trait; the way they reflect light is fundamentally different from how color works most of ... read more

June 14, 2012  For many years, scientists have been pursuing ways to mimic the perplexing capability of the lotus leaf to repel water. Lotus leaves hate water so much that droplets effortlessly roll off the ... read more